The present invention relates to tantalum and niobium metals and methods of forming products from tantalum and/or niobium, such as tantalum billets or niobium billets. The present invention further relates to tantalum billets or niobium billets that have a fine, uniform microstructure including a uniform grain size.
Metal billets, such as tantalum billets are commercially available from a variety of manufacturers. Typically, these billets are defined by their minimum thickness and/or aspect ratio. Typical billets are cylindrical in shape and have a diameter of 2½ inches or more and/or have an aspect ratio of L/D of greater than 0.5. Thus, billets are not metal plates or slabs and are typically an intermediate product formed from an ingot, such as a tantalum ingot. Tantalum billets are then typically further processed by means such as forging into other forms used by a variety of end users for such uses as sputtering targets and the like. The tantalum billets provided to these end users typically do not have a fine and uniform grain size. Instead, commercially produced tantalum billets have a grain structure that varies between the center and edge of the billet. The center of the commercial tantalum billet typically has a microstructure composed of broad bands of larger, elongated grains adjacent to regions of varying fine grain size or of unrecrystallized material. Conversely, the outer portions of the commercial tantalum billets have a relatively fine and uniform grain structure compared to the center of the billet. Thus, products forged from billets having a coarse, non-uniform grain structure may also exhibit a coarse, non-uniform grain structure. For many high performance applications for tantalum such as sputtering targets and chemical energy munition warheads, a non-uniform grain structure has been reported to detrimentally impact product performance (S. I. Wright, G. T. Gray, and A. D. Rollett, Textural and Microstructural Gradient Effects on the Mechanical Behavior of a Tantalum Plate, Metallurgical and Materials Transactions A, 25A, pp. 1025-1031, 1994; C. A. Michaluk, R. O. Burt, and D. P. Lewis, Tantalum 101: Economics and Technology of Ta Materials, Semiconductor International, Vol. 23, No. 8, pp. 271-278, 2000; C. A. Michaluk, Correlating Discrete Orientation and Grain Size to the Sputter Deposition Properties of Tantalum, Journal of Electronic Materials, Vol. 31, No. 1, pp. 2-9, 2002), all incorporated in their entirety by reference herein.
Accordingly, there is a need to provide tantalum and niobium billets having a uniform grain size and preferably made from high purity tantalum and/or niobium. In addition, there is a need to provide methods to make such a tantalum billet or niobium billet.